The present invention relates generally to carbon nitride and more particularly to a method for making C3N4. Still more particularly, the present invention relates to a method for solid-state synthesis of carbo-nitride powders in bulk quantities using halogenated triazines and alkaline metal nitrides as reagents.
In many materials, the manner in which the atoms forming the material are arranged determines many of the physical properties of that material. For example, when carbon atoms are merely assembled randomly, the result is graphite, which is opaque to visible light and relatively soft. The carbon atoms in graphite are not arranged in any ordered fashion, so graphite is unstructured, or xe2x80x9camorphous.xe2x80x9d When the same carbon atoms are subjected to extremely high pressures and high temperatures, however, they align themselves in the crystal structure commonly referred to as diamond. As is well known, diamond is clear to visible light, is very hard, and has a tight, cubic crystal structure. A difference in crystal structure between two materials having identical chemical compositions, such as the difference between graphite and diamond, is referred to as polymorphism.
Various carbon-nitrogen compositions are known in the art. In particular, the preparation of binary, ternary, and quaternary carbo-nitride materials, such as C3N4, Bxe2x80x94Cxe2x80x94N, Alxe2x80x94Cxe2x80x94N, Alxe2x80x94Bxe2x80x94Cxe2x80x94N, Sixe2x80x94Bxe2x80x94Cxe2x80x94N, etc., is an area of current interest. Crystalline forms of these non-oxide lightweight materials are expected to possess a combination of extreme hardness, oxidation resistance and chemical inertness.1-10 The most exciting material in this family is the crystalline form of a particular carbon nitride, C3N4, for which a hardness challenging that of diamond is predicted.1-3 To date, this material has only been postulated and has never been successfully created in commercially meaningful quantities.
Based on its predicted crystal structure, this crystalline form of C3N4 is commonly designated xcex2-C3N4. The xcex2 designation is derived from the crystal structure for xcex2-Si3N4, which is known and is analogous to the expected structure of the desired superhard form of C3N4.
In addition to this potentially superhard xcex2-phase, the existence of xcex1-, cubic, pseudocubic and graphitic (amorphous) polymorph phases of carbon nitride have been recently suggested on basis of calculations11,12 and experiments.13,14 In fact, numerous experimental attempts to synthesize the xcex2-form of carbon nitride using various chemical and physical thin film deposition techniques15 have produced predominantly amorphous materials that lack the desired hardness and have nitrogen contents that are significantly different than the 57 at. % that would be expected for C3N4. For example, several methods of making C2N, C2N2, and C4N5 materials are known in the art. Because of their different carbon:nitrogen ratios, these materials have different crystal structures and thus different mechanical properties from the predicted behavior for xcex2-C3N4.
One possible exception to the foregoing characterization lies in the thin films prepared from single-source precursors by J. Kouvetakis et al.16 and others. Although these films are asserted to have the desired C3N4 stoichiometry, 13C NMR analysis does not confirm the suggested a triazine-based, xcex2-structure for this material. The observation of small xcex1-C3N4 and xcex2-C3N4 or cubic C3N4 crystallites embedded in an amorphous carbon nitride film has also been reported.15,17-21 However, the true nature of these crystallites will remain uncertain until large crystals of this carbon nitride are synthesized and precisely characterized, and their mechanical properties tested. On the basis of the large amount of experimental results available so far, it has been suggested that the physical deposition methods that are known in the art do not yield the desired phases, and certainly do not yield the desired phase in bulk or in commercially meaningful amounts (xe2x80x9cgram amountsxe2x80x9d).20,22 
Hence, a method for producing C3N4 in bulk is desired.
The present invention provides an effective method for producing bulk amounts of a carbon nitride having the composition C3N4 and thus a carbon:nitrogen ratio of 3:4. According to the preferred embodiment, carbo-nitride powders having the desired composition can be produced in bulk quantities using a halogenated triazines and an alkaline metal nitrides as reagents. More specifically, the reagents comprise C3N3X3 where X is a halogen, and M3N where M is an alkaline metal. It has been found that combining the two reagents in dry powdered form, heating them to a temperature above the boiling point of the triazine (C3N3X3) and holding them at an elevated temperature for a predetermined period of time produces bulk quantities of a compound having a C:N ratio of 3:4. As used herein, the designation a-C3N4 refers to a composition having C3N4 stoichiometry and an amorphous (xe2x80x9ca-xe2x80x9d) structure.
The present approach, which is based on fast solid state reactions, is particularly attractive since: (i) it uses the relatively cheap reagents and does not require synthesis of single-source precursors, as in the previously reported preparation of carbo-nitride; (ii) it produces powders with a higher nitrogen content than, for example, the carbon nitride powders of approximately C4N5 stoichiometry described in German Patent DE 197 06 028.5, 199745, and (iii) it may allow the design of reaction routes leading to production of not only binary, but also ternary and quaternary carbo-nitride materials with controlled stoichiometry, morphology, mechanical and electric properties. Ternary and quaternary carbo-nitride materials are those in which one or two, respectively, additional elements are present in the material in addition to carbon and nitrogen.